Polyhydroxyalkanoate-based Multiphase Materials - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 128

e.g. the thermo-mechanical input. The main parameters during melt pro-
cessing will be temperature, residence time, moisture content and atmos-
phere. But, the major problem in the manufacturing of PHA-based products
is their high thermal sensitivity. PHA shows a low degradation temperature
compared to its melting temperature. For instance, PHB homopolymer
presents a narrow window for processing conditions. The PHB thermal 24-31
and thermo-mechanical 32,33 stabilities have been well described in the lit-
erature, demonstrating that the thermal degradation occurs according to a
one-step process, namely a random chain scission reaction. Under extrusion,
increasing the shear level, the temperature, and/or the residential time 34
leads to a rapid decrease in the molten polymer viscosity and its molecular
weight due to macromolecular chain cleavage. In comparison, PHBV co-
polymers are more suitable for the melt process because an increase in HV
content results in lower melting and glass transition temperatures 35 and
thus a wider processing window.
To improve such a drawback, or to give PHAs new properties, a great
number of multiphase materials have been developed, mainly by mixing PHB
or PHBV with other products such as plasticizers, fillers or other polymers.

d n 2 r 4 n g | 9

6.2.2 PHA Plasticization
Many routes were investigated to ease PHB transformation 36 including
plasticization. Many authors have noticed that PHA properties can evolve
when plasticization occurs, e.g. with citrate ester (triacetin). 37-39
Wang et al. tested different plasticizers—dioctyl phthalate, dioctyl seba-
cate, and acetyl tributyl citrate (ATBC)—with PHB. From the DSC measure-
ments, they concluded that only the addition of ATBC leads to an obvious
decline in T g and improves other thermal characteristics. However, ATBC
does little to improve the mechanical properties. 40
The effects of biodegradable plasticizers on the thermal and mechanical
properties of PHBV were studied by Choi and Park 41 using thermal and
mechanical analyses. Soybean oil (SO), epoxidized soybean oil (ESO), dibutyl
phthalate (DBP) and acetyl tributyl citrate (ATBC) were tested as plasticizing
additives. PHBV/plasticizer blends were prepared by evaporating solvent from
solutions. DPB and ATBC were more effective than soybean oils (SO and ESO)
in lowering the glass transition temperatures as well as in increasing the
elongation at break and the impact strength of the films. From the thermal
and mechanical properties of the plasticized PHBV, it could be concluded that
ATBC or DBP are better plasticizers for PHBV than SO and ESO.
From these different studies, it seems that acetyl tributyl citrate (ATBC) is
the most ecient plasticizer for the main PHAs, i.e. PHB and PHBV.

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6.2.3 PHA Nucleating Agents
Crystallization of the PHAs is often slower than desired and has a negative
impact on the final properties of the material. It is thus important to ensure